Any references to methods, apparatus or documents of the prior art are not to be taken as constituting any evidence or admission that they formed, or form part of the common general knowledge.
Worm and gear mechanisms have been known for centuries for use in speed reduction, torque multiplication and general applications where a rotational-to-rotational transmission is required.
The most basic worm and gear mechanism includes a worm of constant pitch thread that meshes with the teeth of a gear or “worm wheel”.
In such a mechanism only one of the teeth is forced by the worm at any one time. That is, the gear and worm contact is restricted to one sweet point of contact that is a sliding contact between each main component. Only at the exact perpendicular point between the gear and the worm is contact made and that point carries all the torque driving force in sliding contact.
More complex worms, which are capable of imparting force along a longer arc between the gear and worm are known and these may be referred to as “helical”, “hourglass” or “globoid” worms. For example, U.S. Pat. No. 2,338,367 to Trbojevich describes a truncated, tapering globoid worm which is designed to drive its gear either forwardly or backwardly.
A problem that may arise in such a worm drive is that of undue wear occurring between the teeth of the gear and the worm.
One approach to addressing the problem of wear is described in European Patent Application No. EP2251564A1 to Meko Seiki Inc. in which the worm gear includes peripheral magnets, each of which hold a ball to a respective socket formed in the wheel. The balls mesh with the thread of the helical worm so that there is a rolling contact between the worm and the gear wheel. It will be realized however that manufacturing a gear wheel with precisely located peripheral magnets is a complex endeavor. Furthermore, the magnets are at risk of becoming demagnetized in which case the balls are no longer held in place. In addition, the balls are subject to considerable shear forces which is undesirable.
Another approach to providing rolling contact between a worm and a gear is set forth in US Patent Publication No. US20060156845 to Tong, wherein the balls circulate along a helical track of the worm and are recirculated through a tunnel formed therethrough. However, the worm drive described in Tong may not be able to transmit high torques from the worm to the gear as would be desirable in a torque multiplier for use as a wrench or a winch.
It is an object of the invention to provide an improved worm drive that is capable of operating at high levels of torque.